Performance Analysis Of A Dc Motor Computer Science Essay

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The aim of this paper is to investigate different available techniques of controlling a Permanent Magnet Brushless DC Motor. In modern era, at every major electrical machine application, a wide range of speed and torque control of the electric motor is required. Undoubtedly, the dc machine fulfills these requirements, but the dc machine requires constant maintenance. The brushless permanent magnet motors do not have brushes and so they require less maintenance only. Brushless dc motors are widely used in applications which require wide range of speed and torque control because of its low inertia, fast response, high reliability and maintenance free. In this study, we have discussed several major procedures to control speed of a DC motor such as; current controlled technique based on the generation of quasi- square wave currents, Programmable Logic Controller (PLC), Support Vector Machine (SVM) Controller, Sensorless Control Technique, Microcontroller and Fuzzy logic controller using Pulse Width Modulation.

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Index Terms- Brushless DC Motor, Pulse Width Modulation (PWM), Programmable Logic Counter (PLC), Support Vector Machine (SVM), Digital Signal Controller (DSC), Fuzzy Logic Controller.

advantages: 1) the quasisquare-wave armature currents are mainly characterized through their maximum amplitude values, which directly controls the machine torque; 2) the position sensor system for the shaft needs only to deliver six digital signals for commanding the transistors of the inverter; 3) the inverter performance is very much reliable, because there are natural dead times for each transistor.

Introduction

Direct current (DC) motors have been widely used in many industrial applications and home appliances. They are used for many speed and position control systems where their excellent performance, ease of control and high efficiency are desirable characteristics. It is designed to run on a DC electric power which is used electrical energy and produce mechanical energy. There are two types of DC motor which is brushed and brushless DC motor. Brushless DC motor (BLDC) is a synchronous electric motor which is powered by DC electricity and which has an electronically controlled commutation system, instead of a mechanical commutation system based on brushes In such motors, current and torque, voltage and rpm are linearly related. In recent decades, research and development on DC motor speed control strategies and their subsequent implementation have been reported. A review of prior work indicates that cost minimization has emerged as the main focus of speed control applications.

I. CURRENT CONTROLLED TECHNIQUE

This procedure is guided to give a simple and efficient modulation control system [1], which allows having good current waveforms. It can be used because of the following

There are two ways to control the phase-currents of a BDCM: 1) through the measurement of the phase currents, which are compared and forced to follow a quasisquare template; 2) through the measurement of the dc link current, which is used to get the magnitude of the phase-currents, Imax.

As the motor is of the brushless dc type, the waveforms of the armature currents are quasisquare. These currents are sensed through current sensors, and converted to voltage signals. These signals are then rectified, and a dc component, with the value of the ceiling of the currents, Imax, is obtained as shown in Fig. 1. This dc signal is compared with a desired reference Iref, and from this comparison, and error signal Ierr is obtained. This error is then passed through a PI control to generate the PWM for all the six valves of the inverter, which

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Power Electronics Project The University of North Dakota Z.Islam, H. Salehfar

are sequentially activated by the shaft position sensor. The torque is directly commanded by Iref.

The control only needs to be in command of one dc current instead of three alternating waveforms. Another advantage of this strategy is that the modulation of the currents can be done using one of the simplest control strategies available: the "triangular carrier modulation strategy" which offers the following additional advantages: 1) the switching frequency becomes defined by the triangular carrier Fig. 2. Stator and rotor's MMF during step change from motor to brake operation. 2) The ability to follow the template with the proposed method becomes quite accurate when triangular carrier is used 3) the hardware implementation is very simple.

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FIG 3. SIMULATED CIRCUIT DIAGRAM

The advantages of this strategy are a) very simple control scheme; b) phase currents are kept balanced; c) current is controlled through a dc component, and hence phase over currents are eliminated. These characteristics allow to use the triangular carrier as a current control strategy for the power transistors, which is simpler and more accurate than other options.

II. PROGRAMMABLE LOGIC CONTROLLER

In this case [3], Toshiba T1-16s PLC is used. The PLC has 8 inputs, phototransistors, and 8 outputs, 6 relay outputs and two transistors which are mainly used to provide clock signal or PWM. The PLC takes an action to switch only two NAND gates at any moment driving the output transistors to be switched off which makes the signal to the NAND gate to be high (15V). The effect on the upper half is that the PWM signal will appear on the output on the NAND gate which is applied to the optocoupler.

Fig. 4: BLDCM Drive System

When the optocoupler is on the MOSFET is driven to the off state and vice versa. In the lower half, the MOSFET will be on at all times (no PWM) during the off period of the PLC transistor output. Once the rotor moves and the PLC receives the new position from the feedback sensors, the current off transistor output is activated (switched on) and new one is switched off to drive a different MOSFET (from the upper or lower half based on the BLDCM sequence of operation). All resistors are selected based on the maximum current consumption of each component.

Fig. 5: Upper Half Driving Signal

The lower half is not assigned to a PWM, soft switching method; however, one pin is always on and the other waits the signal from the PLC. This is illustrated in Fig. 4. The output of the NAND gate is applied to the optocoupler, HCPL-4503, to operate in the pull down mode.

Fig. 6: Lower Drive D.S

The motor runs at a higher speed when low switching frequency is applied. Linearity of the speed curve is observed with all switching frequencies; however the speed tends to be constant when the duty cycle is close to 100%. The on and off delays from the driving transistor of the PLC made the operation to consume more current and reduces the speed because of the on/off delay.

III. SUPPORT VECTOR MACHINE CONTROLLER

The Support Vector Machines are used for classification task of only two-class problems [2]. In Support Vector Machine, a separating hyperplane is used. This hyperplane of Support Vector Machine is obtained by calculating the maximum distance to the closest data points of the training set.

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Power Electronics Project The University of North Dakota Z.Islam, H. Salehfar

These closest data points of the training set are named as Support Vectors. The data points of the training set may be linearly separated into the input space. These data points of the training set can be transformed to a High Dimensional Space by using a nonlinear transformation in this status.

Fig. 7: SVM inverse modeling and control blocks

In the figures, the SVM inverse modeling and control blocks are given respectively. In these simulation studies, mentioned blocks are used for SVM control of dc motor. Then, SVM generalization process is realized by using the inverse modelling block given in figures. The obtained control results for the nominal values of system parameters K = 1/J = 100 and a = B/J = 2 (J = 0.01 kgm2 and B = 0.02 Nms/rad) are shown in figure. Beside, + / - 15 values of the saturation function are used for limiting the outputs of all controllers in the simulation study.

When, the zero of controller (a) is constant value, response of the PI is not distorted as long as the value of K decreases in from 100 to 55. Because, the system response of PI slows down as the gain (K) is decreased. Therefore, the system response of SVM fixes as the gain (K) is decreased.

Fig.8: K = 100 and a = 2 nominal parameter values are used in the SVM controller and PI controller for the system response.

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Fig.9: The comparing the system responses of PI and SVM controllers for parameters for the K = 100 and a = 0.4.

The dc motor speed control performance of designed SVM controller is compared with the linear PI controller's. The dc motor speed control performance of designed SVM controller is very high. While the dc motor speed control performance of linear PI controller is degredation, the SVM controller's is robust versus to changes of controller parameter values. So, control performance of designed SVM controller is beter than linear PI controller's. Nevertheless, the rise and settling time obtained by the SVM controller system can be considered to be the best values.

IV. SENSORLESS CONTROLLER

BLDC motors can be commutated by monitoring the back EMF signals instead of the Hall sensors [6]. Every commutation sequence consists of one motor winding driven high, one driven low and the third winding left floating. By measuring the voltage in the floating winding, the zero crossing point (ZCP) can be found. The ZCPs occur when the back EMF is zero, during which time the phase voltage equals half of Vdc. When the ZCP is detected the commutation signal is generated by the digital signal controller.

The back EMF technique is very handy when it comes to cost and complexity. Since back EMF is proportional to speed it is impossible to identify the rotor at standstill. However, for applications where operation near zero speed is not required the motor can be started in open loop until there is sufficient back EMF to detect the zero cross point. The importance of sensorless control for electronic commutation cannot be over emphasized and should be utilized if possible. Obvious reasons in favor of sensorless control are cost, reliability and ability to withstand harsh environments.

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Power Electronics Project The University of North Dakota Z.Islam, H. Salehfar

is monitored to sense the rotor magnet position. Each phase detection circuit is connected to an ADC channel. ADC of the digital signal controller, the back EMF zero crossing point is determined. Based on zero crossing of back EMF signals, motor commutation is decided in the firmware.

Fig.10: Phase terminal voltage and ZCP

The central processing unit is implemented by a digital signal processor. A digital signal processor (DSP) provides high speed, high resolution and sensorless control algorithms at lower system costs. The driving circuit is implemented to drive six power MOSFETs controlling the BLDC motor.

Fig.11: Control Block Diagram

To drive the MOSFETs, a high speed MOSFET driver IC is used. The MOSFET driver chosen for the design is the IR2101 package from International Rectifier. The single 8-pin package has both a high and low side gate driver. As such, one IR2101 is capable of driving one MOSFET pair. Since the motor controller is intended for a three phase motor there are three identical gate driver circuits. The recommended connections given by the IC's data sheet are used to interface the inverter to the driver. Each gate driver circuit contains a bootstrap circuit that consists of a diode, capacitor and gate resistors.

Fig.12: Gate Driver Circuit

The PWM signals drive three gate drivers, which in turn, drive the three phase bridge inverter circuit connected to the three motor windings. Two motor windings are energized at any one time in exact synchronism with the rotor motion whilst the third winding is left open. In this study the BLDC motor is run without sensors so the back EMF on the unexcited winding

Fig.13: Current Detection Circuit

Variable speeds were achieved through varying the duty cycle of the PWM signal at no load. Since the amplitude of back EMF is proportional to speed, it is also impossible to detect the back EMF zero crossing point when the motor is at standstill. Therefore, a suitable starting procedure must be employed in the firmware. The implemented sensorless control technique is ideal for use in appliance, automotive and industrial applications where operation near zero speed is not required.

Fig.14: The speed verses duty cycle

V. MICROCONTROLLER

Microcontroller based speed control system consist of electronic component, microcontroller and the LCD [7]. In this paper, implementation of the ATmega8L microcontroller for

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Power Electronics Project The University of North Dakota Z.Islam, H. Salehfar

speed control of DC motor fed by a DC chopper has been investigated. The chopper is driven by a high frequency PWM signal. Controlling the PWM duty cycle is equivalent to controlling the motor terminal voltage, which in turn adjusts directly the motor speed. This work is a practical one and high feasibility according to economic point of view and accuracy.

The circuit is based on PWM technique. ATmega-8's timer portion has this special feature. By adjusting register values duty cycles can be controlled. When motor run at 70% of duty cycle, the techogenerator gives a Voltage corresponds to that speed. Now if any load occurs, desired speed will be decreased. Hence the voltage drop will be less. This voltage is fed into the ADC of microcontroller. By comparing the previous value microcontroller can sense the decrease in the speed. After sensing the load condition it will start increasing its duty cycle until it reaches the desired speed. . During overload condition Microcontroller will try to reach the desired speed by increasing duty cycle. But if at the maximum duty cycle it fails to run it will show a message to the user through A LCD panel. The message indicates OVERLOAD. Now user can run the machine again at desired speed by decreasing the load.

Fig.15: Motor Control section

A pulse with fixed frequency is generated by the microcontroller, which is fed to the base of transistor. Transistor acts here as a switch. The output voltage of the motor is dependent on the amount of the on time of the transistor. The more time transistor remain on more the voltage will produce. A Freewheeling diode is used for back e.m.f. protection given to other portion. Output voltages at different duty cycles has found by varying the duty cycle controller register OCR (output compare register) shown in Figure. The system shows the immediate response of maximum output voltage when 100% duly cycle is achieved. The voltage drop across the potentiometer fed to ADC of the microcontroller. According to the ADC value, microcontroller will take decision whether pulse width needs increment or decrement.

Fig.16: Duty cycle vs. output voltage

VI. FUZZY LOGIC CONTROLLER

The fuzzy logic foundation is based on the simulation of people's opinions and perceptions to control any system.[14] One of the methods to simplify complex systems is to tolerate to imprecision, vagueness and uncertainty up to some extent. An expert operator develops flexible control mechanism using words like "suitable, not very suitable, high, little high, much and far too much" that are frequently used words in people's life. Fuzzy logic control is constructed on these logical relationships. There is a strong relationship between fuzzy logic and fuzzy set theory that is similar relationship between Boolean logic and classic set theory.[11]

Fig.17:Control Block Diagram

FLC uses different strategies for motor speed control. FLC process is based on experiences and Linguistic definitions instead of system model. It is not required to know exact system model to design FLC. In addition to this, if there is not enough knowledge about control process, FLC may not give satisfactory results. The goal of designed FLC in this study is to minimize speed error. The bigger speed error the bigger controller input is expected. In addition, the change of error plays an important role to define controller input. Consequently, FLC uses error and change of error for linguistic variables which are generated from the control rules.

System speed comes to reference value by means of the defined rules. According to this rule, if error value is negative

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Power Electronics Project The University of North Dakota Z.Islam, H. Salehfar

large and change of error value is negative large than output, change of alpha will be positive large. To be calculated FLC output value, the inputs and outputs must be converted from 'crisp' value into linguistic form. Fuzzy membership functions are used to perform this conversion.

Fig.18: The Rule Database

The fuzzy logic controller is able to sensitiveness to variation of the reference speed attention. [8] The speed control of dc motor showed the controller gains optimal performance. The controller achieved to overcome the disadvantage of the use conventional control sensitiveness to inertia variation and sensitiveness to variation of the speed with drive system of dc motor.

VII. CONCLUSION

PWM of a signal source involves the modulation of it duty cycle to either convey information over a communication channel or control the amount of power sent to a load. Motors may be able to operate at lower speed if we control them with PWM. When we use an analog current to control a motor it will not produce significant torque at low speeds. These techniques have several advantages and disadvantages for controlling the speed of a DC motor. However, still we need more efficient procedure to control DC motor speed by comparatively less costly equipments.